The present invention relates to a tunable coherent light source comprising an optical parametric oscillator device for the visible and near infrared region based on stoichiometric ferroelectric crystal material as the parametric gain medium pumped by pulsed solid state lasers.
Optical Parametric Oscillators (OPOs) are used as sources of tunable coherent radiation. An OPO includes a non-linear optical crystal and an optical resonator cavity. The non-linear optical crystal converts an input laser wave (referred to as a “pump” beam) into two output waves called the signal beam and the idler beam. The wavelengths of the signal and idler beams are determined by the pump frequency, the angle the pump wave vector makes with respect to a crystal axis of the non-linear optical crystal, and the phase matching properties of the non-linear optical crystal. The optical resonator cavity resonates at least one of the signal and idler beams.
Electric field poling is used to fabricate periodically-poled quasi-phase-matched ferroelectric crystals which are used to produce efficient and compact all solid-state OPOs. However, ferroelectric material is known to be highly susceptible to photo refractive damage and visible-induced infrared absorption such that OPOs using ferroelectric material as the non-linear optical crystal are confined to operating in the infrared region. Near-infrared OPOs using LiNbO3 and LiTaO3 as the non-linear optical crystal are used for low power applications. However, refractive damage limits their further development in high power OPOs. Accordingly, compact high repetition-rate high power OPOs broadly tunable in the visible and near-infrared are still not available.
U.S. Pat. No. 6,211,999 discloses the use of near-stoichiometric LiTaO3 (SLT) with an increased optical damage threshold. Periodically-poled near stoichiometric LiTaO3 (PPSLT) is applied in efficient second harmonic generation and sum frequency mixing to the blue and green regions. However, the devices described in U.S. Pat. No. 6,211,999 also suffer from photo refractive damage and light-induced absorption under intense radiation of >15 MW/cm2. Similar ferroelectric crystals made of MgO doped SLT and MgO doped stoichiometric lithium niobate have similar problems.
OPOs typically include a plurality of periodically poled inverted domain structures, each having a different period. Each section or segment is used to generate different signal and idler wavelengths. Another problem with known periodically-poled crystal devices is that many segments of slightly different periodicity are required to provide a broad continuously tunable range.
Commercially available high power tunable lasers include dye lasers and Ti: sapphire lasers. Dye lasers use chemicals that are potentially toxic and possible cancer agents. Ti: sapphire lasers have a limited tuning range and are difficult to time.